It is well known to use pressure or vacuum filter leaves inside of a tank or pressure vessel for liquid-solid separation. Typically, a pressure or vacuum filter leaf is two-sided and includes a filter medium, a spacing screen, a chamber frame, a binder or closure and an outlet. The filter medium forms both of the outside filtering surfaces of the filter leaf. A chamber through which liquid flows is defined on the inside of the filter leaf by a chamber frame positioned between both of the filtering surfaces. Typically, both the filter medium and the chamber frame are constructed of a wire mesh configuration. If desired, a spacing screen can be positioned between the chamber frame and each of the filter mediums. The spacing screen may also be constructed of wire cloth or a thin perforated sheet of metal or plastic. A binder or closure which is shaped and configured to envelop the entire outside edge of the leaf holds the leaf together and provides the rigidity necessary for the leaf. The filter medium can be attached to the filter leaf by conventional welding, crimping, bolting or riveting methods or, in the case of non-metallic materials, as a bag which envelops the entire leaf. An outlet is provided which allows discharge of the filtered liquid through an outlet manifold. The outlet may be at or near the center of the leaf, or alternatively along the periphery of the leaf.
In use, the filter leaves of the above-noted construction are positioned within a tank such as a pressure vessel and are connected to a manifold type discharge. The material to be separated into its liquid and solid components is allowed to flow into the tank under pressure and separation takes place on the filtering surfaces of the pressure filter leaves.
Vacuum filter leaves, which are similar in construction to pressure filter leaves, function in the same manner as do pressure filter leaves except that they employ a vacuum which is applied to the chamber of the leaf instead of the pressure which is applied to the outside filtering surfaces.
In both pressure and vacuum filter leaves the filtered liquid flows through the leaf, into the chamber therein and out the discharge manifold. If desired, a "filter aid", such as diatomaceous earth, is placed into the tank so as to coat the filter leaves before commencement of filtration. The "filter aid" helps to regulate the filtration of particles having a predetermined size. Operation of the pressure or vacuum filter may be on a continuous or batch type operation.
After a suitable amount of filtration has occurred or periodically, the filter leaves are cleaned, in either of two methods, of the particulant matter which cakes or deposits on the filtering surfaces. According to a wet cake discharge method, the material collected on the leaves can be discharged by allowing pressure to drop followed by spraying, i.e., sluicing the filter leaves with a liquid to wash off the solids from the filter leaves. Alternatively, in the dry cake discharge method, the cake is first dried and then blown off from the filtering surfaces with air. After the air is blown on the outside of the leaf, the leaf can also be brushed, scraped or vibrated so as to remove caked material.
Alternatively, pressure or vacuum filter leaves can be constructed in the form of sectors which are placed side by side to form a 360 degree circle around a central shaft through which the vacuum is supplied. Vacuum disc filter sectors are sometimes constructed of plastic with a plastic bag serving as the filter medium. Typically, there may be one or preferably more such circles of sectors on the same central outlet. The filter rotates in a vat containing the liquid and solid to be separated, with the lower half of the filter submerged in the vat. As the filter rotates, solids are drawn against the filter medium of each submerged sector. Liquids pass through the sector and out along the central outlet. As is the case with pressure filter leaves, the sectors periodically and in some cases during each complete rotation of the sector, must be cleaned to dislodge solids or particles from the filter medium. These solids can be removed when the sector rotates out of the vat and before the sector is again submerged. This cleaning can be performed with gas, steam or liquid.
The filter leaves constructed as described above have disadvantages which result from that type of construction. Since the filter medium, chamber frame and spacing screens are typically of an interwoven, preferably wire mesh configuration, these meshes provide numerous tortious paths which tend to clog with particles which are to be separated from the liquid. Also, such meshes do not readily or easily permit removal of any cake buildup. Likewise, "filter aid" material often becomes trapped within the mesh and after excessive trapped buildups may result in reduced efficiency of filtration as well as offering resistance to removal of the "filter aid". Additionally, such filter leaves are not suitable for "back-flushing" removal of the caked or deposited material on the filtering surfaces of the filter leaf, i.e., by flowing an incompressible and/or compressible fluid in a direction opposite to the flow of separated liquid, since these filter leaves tend to blow-out during "back-flushing". Also, such filter leaves are often damaged or wear quickly because of the use of brushes or scrapers to clean the filtering surfaces and, furthermore, are particularly subject to damage in the event a doctor blade is used to clean the filtering surface.
Although filter screens are known which employ single wedge wire screens, such filter screens have not replaced known pressure filter leaves as described above. In particular, it is known from U.S. Pat. Nos. 1,177,313; 2,346,885 and 3,941,703 to employ a single flat or cylindrical wedge wire screen as the filter medium for straining well water or for dewatering or washing coal ores and the like. Wedge wire screens are described in greater detail in U.S. Pat. Nos. 3,169,111 and 3,716,144 which illustrate various constructions of wedge wire screens. However, as noted in U.S. Pat. No. 3,941,703 such flat wedge wire screen surfaces are not preferred for high pressure filtration.